The implementation in 2003 of the EU Water Framework Directive (2000/60IEC) in England and Wales necessitates addressing water quality issues at the river catchment scale, In the first instance, the overall aim of the Directive is to achieve 'good ecological and chemical status' in all surface and ground water bodies by 2015. Principally due to the presence of elevated metals concentrations, abandoned mine water pollution has been identified by the Environment Agency (England & Wales) as a potentially major impediment to meeting this aim. Particular uncertainty surrounds the importance of diffuse sources of pollution from abandoned mines, the sources and quantification of which have been poorly studied to date. The overall aim of this study was therefore to investigate the importance of diffuse sources of ,,- metals pollution associated with a former-mining district. Within this overall aim, primary objectives included an assessment of the role of diffuse sources under varying hydrological conditions, and an investigation of the potential significance of riverbed sediment as a source and sink o~ metal pollutants. The study site, the River West Allen catchment, Northumberland, forms part of the Alston Block in the north Pennine orefield, the mineral veins of which are largely hosted in Carboniferous Limestone strata. The area was principally worked for zinc (and lead) for over 300 years. As a result, the river is now impacted by three distinct point discharges from these now long abandoned lead-zinc mines. Preliminary reconnaissance surveys illustrated that the pollution arising frpm these drainage adits primarily influences the river's total zinc concentrations, which heavily exceed European statutory environmental quality standards (EQS) throughout much of the catchment. Thus, the sources, fate and dynamics of zinc was the principal focus ofthe work. In this study, synchronous monitoring of aqueous hydrochemistry, flow measurements, and river bed sediment geochemistry, has been -routinely carried out under varying flow conditions for over a year. This has enabled the calculation of the contaminant metal loads at point mine water discharges and selected river locations, under differing hydrological conditions, thus establishing the overall impact of point mine waters within the catchment, and allowing the derivation of contributions from diffuse mine water pollution to the instream totals. In addition, sampling and analysis of river bed sediments (including sequential extraction procedures) has allowed an assessment of the potential importance of such materials as a contributor to diffuse metals pollution. The results of the investigation show that during low flow conditions point source mine water pollution contributes significantly to zinc loadings in the river. In fac~ the total zinc loading . - - ~ - of pojnt sources far exceeds that of the zinc loading in the river downstream of these discharges in low flow conditions (by up to 440%), suggesting considerable attenuation of zinc within the river channel. At all sample sites in the river EQS values are exceeded by up to a factor of 37. Under higher flows instream zinc concentrations exceed EQS values by up to a factor of 57. Point source contributions to total instream zinc loading become far less significant, falling to as little as just 10%. Thus diffuse sources of zinc pollution (i.e. those unaccounted for in the zinc mass balance calculated from point source and instream loadings) account for up to 90% of the instream zinc load, highlighting the dynamic importance of diffuse inputs. Possible diffuse sources include 1) sediment resuspension, due to scouring of metal contaminated river-bed sediments, .q.fld possible subsequent metal remobilisation, 2) direct groundwater input via the hyporheic zone and 3) surface run-off from mine spoil. Of these diffuse inputs the resuspension and possible remobilisation of heavily contaminated sediment has been .investigated in detail here, and appears to be an important potential source of increases in the instream zinc loading during high flow events. The instream concentration and loading of total suspended sediment increases approximately 30-fold and 2,200-fold, respectively, from low to high flow, suggesting sediment resuspension is a key process in releasing zinc into the water column. Zinc concentrations within river-bed sediments typically ~ange from 6,000 - 75,000 mg/kg. These values far exceed interim threshold concentration limits (currently adopted by the Environment Agency) by up to a factor of 600. The majority of this zinc is deemed 'potentially available', as approximately 50% of the total zinc concentration IS associated with carbonates and hydrous ferric oxides. Therefore, changes within pH or redox conditions within the water column may have the potential to remobilise particulate zinc. This work emphasises the potential importance of diffuse sources of mine water pollution in river catchments in former mining districts. In the River West Allen the proportional significance of such point and diffuse sources to total instream zinc loading differs under varying hydrological conditions, and the findings have implications for the management of such pollution issues more widely. For example, it raises questions as to whether treatment of point mine water sources alone will result in measurable environmental benefits in terms of meeting the objectives of the Water Framework Directive. In the River West Allen catchment, while remediation of the point sources of mining pollution would lead to improvements in low flow water quality, the legacy of mining pollution in sedimentassociated metals, and possibly other sources of diffuse metals contamination, may continue to provide an ongoing source of diffuse metal pollution at higher flows.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:485509 |
| Date | January 2008 |
| Creators | Gozzard, Emma |
| Publisher | University of Newcastle Upon Tyne |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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